The EMnify Mobile Core will announce the recommended DNS server during the PDP context activation. Therefore, please ensure your device does not have a pre-configured DNS server.
When connected outside the private static IP address space (i.e. outside of a VPN tunnel), in order to establish data connection between your server and the device, please add the following IP to the white-list of your server's firewall - 188.8.131.52. You shall see incoming connection requests from this IP, which is associated publicly with your device's private static IP.